Re: [PATCH 1/2] dt-bindings: firmware: arm,scmi: Document arm,poll-transport property

[Marek Vasut <marek.vasut@mailbox.org>]

On 12/2/25 3:52 PM, Sudeep Holla wrote:

Hello Sudeep,

>>> While I was going through the SCMI spec, DEN0056F , page 209 , section "4.1
>>> Shared memory based transport" , bullet • Completion interrupts, I found it
>>> explicitly states:
>>>
>>> "
>>> This transport supports polling or interrupt driven modes of communication.
>>> In interrupt mode, when the callee completes processing a message, it raises
>>> an interrupt to the caller. Hardware support for completion interrupts is
>>> optional.
>>> "
>>
>> Oh, yes...I knew that...it is just that till now, no systems were really
>> ever developed that lacked the completion IRQ as a whole, it was, till now,
>> more of a case of having the capability NOT to use it selectively at runtime
>> and instead use polling when wanted (like for clock ops in ISR context)
>>
> 
> Indeed.
> 
>> I am not sure what is the reason why this only-polling scenario was never
>> supported in the HW description, this indeed pre-dates my work on SCMI....
>> ...I would/will check with Sudeep, when he's back, what are the reasons for
>> this (if any)...
>>
> 
> As you mentioned earlier, no platform has required this before. I’m fine with
> adding it, but we need to be more explicit about what it implies for SCMI. The
> transport may be shared with other system components, and enforcing polling
> for SCMI while the same transport generates interrupts for another user could
> lead to issues.

How do you imagine this -- a transport shared with other components, one 
which does generate IRQs and one which does not -- would look like ? Can 
you think of an example ?

> Clearly defining these constraints would be helpful. It may also be useful to
> note that this is primarily intended for mailbox transports, if that’s
> accurate. Alternatively, we could keep the DT binding definition broader but
> emit warnings when a transport other than mailbox is used. That approach might
> make it easier to move forward.

DEN0056F refers to this polling mode in Shared memory based transports, 
that can be other than mailbox transports, it includes e.g. SMC or OPTEE 
transports.

I don't think a warning is justified, if the behavior follows the 
specification. But I do agree the behavior is ... suboptimal.

-- 
Best regards,
Marek Vasut